The present invention relates to a method of and an apparatus for driving a laser beam source to apply a laser beam to an image recording medium, such as a photographic film for recording an image thereon. More particularly, the present invention relates to a method of and an apparatus for driving a laser beam source to energize/de-energize laser oscillation for exposing a subject or original while supplying a bias current of a level lower than a threshold level of the laser beam source to the laser beam source in order to eliminate image density irregularities which would otherwise be caused by a droop of the laser beam source. In the present invention, the bias current is supplied to the laser beam source at a position in front of a leading edge of the subject for preventing a recorded or reproduced image of the original from being fogged in its entirety or losing edge sharpness, so that high-quality images can be recorded or reproduced.
In printing and platemaking industries, image scanning recording/reproducing systems are widely used for electrically processing image information of subjects or originals to produce film plates in order to simplify the operation process and increase the image quality.
Such an image scanning recording/reproducing system basically comprises an image reading device and an image recording device. In the image reading device, image information on a subject which is fed in an auxiliary scanning direction is scanned in a main scanning direction by a photoelectric transducer which converts the image information to an electric signal. Then, the image information which has been photoelectrically converted by the image reading device undergoes various processes such as for gradation correction, profile emphasis, and the like dependent on the platemaking conditions in the image recording device. Thereafter, the image information thus processed is converted to a light signal represented by a laser beam, which is applied to an image recording medium made of a photosensitive material such as a photographic film to record the image information thereon. The image recorded on the image recording medium is then developed by an image developing device, and the developed image recording medium is used as a film plate for printing operation.
Laser diodes are widely used as the laser beam source for exposing the image recording medium with the laser beam since the laser diodes are small in size, low in cost, and highly reliable in operation.
FIG. 1(a) of the accompanying drawings shows a conventional driver circuit for a laser beam by way of example. A laser diode LD is connected between the source terminal of an FET 2 and ground, and a resistor 4 is connected between the drain terminal of the FET 2 and a power supply +V. The gate terminal of the FET 2 is supplied with a pulse signal Ls of a rectangular wave (see also FIG. 1(b)) through a buffer 6 for driving the laser diode LD. The laser diode LD is energized only for those periods in which the pulse signal Ls is positive. The timing of light emissior from the laser diode LD is therefore accurately controlled by the pulse signal Ls for exposure of a photographic film or the like.
When the laser diode LD is energized at T.sub.2 (FIG. 1(b)) by the pulse signal Ls after a pulse-free interval of about 300 .mu.s indicated at (i) in FIG. 1(b), the amount of light emitted from the laser diode LD immediately after the laser diode LD is energized becomes a few % greater than a desired target amount of light LP. This phenomenon is generally called a "droop". It is known that the droop is caused by temperature-dependent characteristics of light emission efficiency of the laser diode LD. A photographic film which is exposed to a laser beam with a droop has a higher image density when exposure is commenced, with the result that the reproduced image suffers density irregularities.
An attempt has been made to solve the above problem by supplying the laser beam source at all times with a bias current I.sub.1 (which causes the laser beam source to emit an amount of light P.sub.1) which is lower than a threshold current Ith (which enables the laser beam source to emit a threshold amount of light) as shown in FIG. 2, for thereby reducing a change in the temperature of the laser diode LD at the time of emitting the laser beam. This method can be effected by inserting a bias resistor (not shown) between the anode of the laser diode LD and the power supply +V shown in FIG. 1(a).
However, the above method of reducing temperature changes has the following disadvantages: As shown in FIG. 3(a), when a single photographic film S is exposed to three subjects or original images 8, 10, 12, exposure to the image 8 is effected by scanning an area A x B on the film S with a laser beam having an amount of light P.sub.2, exposure to the image 10 is effected by scanning an area A x C on the film S with a laser beam having the same amount of light, and exposure to the image 12 is effected by scanning an area A x D on the film S with a laser beam having the same amount of light. Therefore, a hatched area on the film S is exposed to the laser beam three times with the bias current I.sub.1 (i.e., the amount of light P.sub.1). The exposure ratio of the hatched area is 3.times.P.sub.1 /P.sub.2 which is of a value of 3% or greater. Consequently, the hatched area on the film S, which should not be exposed to the laser beam, is exposed many times with the low-level laser beam P.sub.1 emitted by the bias current I.sub.1. As a result, the scanning region including the reproduced images may be fogged or the front edge of the reproduced image corresponding to the subject image 8 may be blurred due to the lack of desired sharpness, as shown in FIG. 3(b).